The world’s rivers being polluted by medicines and pharmaceutical products poses a "threat to environmental and global health", according to a recent report.
The University of York study, published on Monday and among the most extensive ones undertaken on a global scale, detected paracetamol, nicotine, caffeine, and epilepsy and diabetes drugs in rivers worldwide, reports the BBC.
The researchers sampled water from more than 1,000 test sites in over 100 countries.
They found that rivers in Pakistan, Bolivia and Ethiopia were among the most polluted, while ones in Iceland, Norway and the Amazon rainforest fared the best.
The impact of many of the most common pharmaceutical products found in rivers is still largely undetermined.
It is, however, well established that dissolved human contraceptives can impact the development and reproduction of fish, and scientists fear the increased presence of antibiotics in rivers could limit their effectiveness as medicines.
On the whole, more than 25% of the 258 rivers sampled had what are known as "active pharmaceutical ingredients" present at a level considered unsafe for aquatic organisms.
"Typically, what happens is, we take these chemicals, they have some desired effects on us and then they leave our bodies," Dr John Wilkinson, who headed the research, told BBC News.
"What we know now is that even the most modern efficient wastewater treatment plants aren't completely capable of degrading these compounds before they end up in rivers or lakes."
The two most frequently detected medicines in rivers were carbamazepine, which is used to treat nerve pain and epilepsy, and metformin, used to treat diabetes.
The researchers also found high concentrations of caffeine (coffee) and nicotine (cigarettes) as well as paracetamol.
In Africa, artemisinin—used in anti-malarial medicine—was also found in high concentrations.
The impact of the presence of such pharmaceuticals in rivers is likely to be negative, said Dr Veronica Edmonds-Brown, an aquatic ecologist from the UK's University of Hertfordshire.
Individual tests have to be done with each medicine and there are relatively few studies, she told BBC News.
"This is only going to get worse as we are increasingly using pharmacological solutions to any illness whether physical or mental."
The report says the increased presence of antibiotics in rivers could also lead to the development of resistant bacteria, damaging the effectiveness of pharmaceuticals and ultimately posing "a global threat to environmental and global health".
The most polluted sites were mostly in low- to middle-income countries, and in areas where there was sewage dumping, poor wastewater management and pharmaceutical manufacturing.
"We have seen contaminated rivers in Nigeria and in South Africa with very high concentrations of pharmaceuticals and this is basically down to the lack of infrastructure in wastewater treatment," said Dr Mohamed Abdallah, associate professor of Emerging Contaminants at Birmingham University.
"This is most concerning because you have the most vulnerable populations with the least access to healthcare exposed to this."
One of the few measures that might have an effect right now is the proper use of medicines, said Dr Wilkinson, the lead author of the report.
That means making it harder to get hold of medicines like antibiotics as well as tighter restrictions on doses.